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Microbe‐Human Interactions:Contact, Infection and Disease
• Host• Any organism that harbors another organism or particle (virus, prion)
• Symbiosis‐ An association between 2 species (“living together”)
– Mutualism• Both members benefit from relationship
• E. coli produce useful products (Vit K) in our large intestine
– Parasitism• One member benefits, one member is harmed
– Commensalism• One member benefits, one member is not benefited nor harmed
• Microbes on our skin utilize skin products
Host-Microbe Relationships: Symbiosis
E. Coli in Lg. intestines
Giardia in intestines
Demodex folliculorum in hair follicle
Microflora
• Resident Microflora• Microbes always present on or in the body
• Transient Microflora• Microbes present for shorter periods of time (minutes to months)
• Adult human body consists of:
– 10 trillion (10 13) eukaryotic cells‐Human cells
– 100 trillion (10 14) prokaryotic cells‐Bacteria cells
– We have 10 times as many prokaryotic cells vs. our own cells!!
Resident Microflora
• Which areas harbor microflora?
• Which body tissues, organs and fluids are usually microbe‐free?
The Absence of ResidentMicroflora can have Harmful Effects
• Cattle Studies:– Enlargement of cecum
– Vitamin deficiency
– Underdeveloped immune system
Microbial Antagonism:• Normal biota are unlikely to be displaced by
incoming microbes• Limited number of attachment sites• Chemical or physiological environment created
by resident biota is hostile to other microbes
•Normal biota is beneficial, or at worst, commensal to the host in good health with a functioning immune system
Resident Microbial Compete with Possible Pathogens
First Acquiring Resident Microflora
• Mother’s birth canal
• Mother’s breast milk
• Bottle‐feeding
• People
• Air
• Surfaces
• The only time that humans are sterile is when they are in the womb (in utero)
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Areas of the Body that are usually Sterile
• Circulatory system• Organs• Glands• Lungs• Sinuses• Middle and inner ear• Brain• Internal eye• Muscles
• Blood
• Urine in the kidneys, ureters, and bladder
• Cerebrospinal fluid
• Saliva before oral cavity
• Semen before the urethra
Contact to Disease1. Contact: Microbes are present
2. Colonization: The presence of bacteria on body surface (skin, mouth, intestines, airway) without causing disease
3. Infection (Infestation‐larger parasites)
Multiplication of microbes (Microbes penetrate host defenses, enter tissue and multiply)
4. DiseaseDisturbance in normal homeostasis
1. doesn’t usually lead to 3. and 4.
True vs. Opportunistic Pathogen
True pathogen Causes disease in healthy individuals
Associated with a specific and recognizable disease
Opportunistic pathogen Causes disease in immune compromised host
Gain access (injury) to sterile regions
Staphylococcus aureus
Vibrio cholerae
Opportunistic MicrobesOpportunists usually do not cause disease unless the “opportunity” arises.
Conditions that opportunists can flourish: Failure of the host’s normal defenses
Immunocompromised populations
Intro of the organism into unusual body sites E. coli normal in gut but not urinary tract
Disturbances in normal microflora Yeast infection after antibiotic use. Why?
Pathogens, Pathogenicity and Virulence
• Pathogen: Disease causing agent
• Pathogenicity: The ability to cause disease
• Virulence: The degree of pathogenicity
• Virulence of a microbe is determined by its ability to
- establish itself in a host
- cause damage
• Virulence factor: any characteristic or structure of the microbe contributes to its ability to establish itself in the host and cause damage
The Progress of an Infection
Pathogen needs to become established by being successful at the following:
1. Portals of entry2. Attachment3. Surviving host defenses4. Causing disease5. Portals of exit
If you stop a pathogen from being able to do any of these steps they will not be able to infect the host
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• Portal of entry: the route that a microbe takes to enter the tissues of the body to initiate an infection
• Exogenous: microbe originating from a source outside the body from the environment or another person or animal
• Endogenous: microbe already existing on or in the body from normal biota or a previously silent infection
The Progress of an Infection:1. Portal of Entry
The Progress of an Infection:1. Portals of Entry
• The majority of pathogens have adapted to a specific portal of entry
• Usually if pathogens enter the “wrong” portal, they will not be infectious
• Inoculation of the nasal mucosa with the influenza virus will result in infection, but if the virus contacts the skin, no infection occurs
The Progress of an Infection: 1. Portals of Entry
• Occasionally, an infectious agent can enter by more than one portal
• Mycobacterium tuberculosis can enter through both the respiratory and gastrointestinal tracts
• Streptococcus and Staphylococcus can enter through the skin, urogenital tract, and the respiratory tract
Infectious dose (ID)• The minimum number of microbes necessary to cause an
infection to proceed• Microorganisms with smaller infectious doses have greater
virulence
- ID for rickettsia is a single cell- ID for tuberculosis and beaver fever is about 10 cells- ID for gonorrhea is 1,000 cells- ID for typhoid fever is 10,000 cells- ID for cholera is 1,000,000,000 cells
The Progress of an Infection: 1. Portals of Entry
- Gain a stable foothold on host tissues
- Dependent on binding between specific molecules on both the host and pathogen
- Pathogen is limited to only those cells (and organisms) to which it can bind
- Firm attachment is almost always a prerequisite for causing disease since the body has so many mechanisms for flushing microbes from tissues
The Progress of an Infection:2. Attachment/Adhesion
Structures
– Capsules
– Pili or fimbriae
– Hooks
– Spikes
The Progress of an Infection:2. Attachment/Adhesion
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•Microbes not established as normal biota will likely encounter the host immune defenses when first entering
•Phagocytes: cells that engulf and destroy host pathogens by means of enzymes and antimicrobial chemicals
The Progress of an Infection:3. Surviving Host Defenses
WBC engulfing S. cerevisiae
•Antiphagocytic factors:• Virulence factors that help pathogens avoid
phagocytes• Leukocidins: kill phagocytes; Streptococcus and
Staphylococcus• Slime or capsule: makes it difficult for the
phagocyte to engulf the pathogen; Streptococcus pneumoniae and Salmonella typhi
• Some bacteria survive inside the phagocyte; Legionella, Mycobacterium
The Progress of an Infection:3. Surviving Host Defenses
• Virulence factors are adaptations a microbe uses to establish itself in a host
• Three ways that microorganisms cause damage to their host
A. directly through the action of enzymesB. directly through the action of toxins (both
endotoxins and exotoxins)C. indirectly by inducing the host’s defenses to
respond excessively or inappropriately
The Progress of an Infection:4. Causing Disease
•Exoenzymes - enzymes secreted by microbes that break down and
inflict damage on tissues- Often dissolve the host’s defense barriers to promote
the spread of disease to other tissues
•Examples of exoenzymes- hyaluronidase: digests the ground substance that
cements animal cells together- coagulase: causes clotting of blood or plasma
The Progress of an Infection:4. Causing Disease‐Enzymes
The Progress of an Infection:4. Causing Disease‐Exoenzymes
•Toxin: a specific chemical product of microbes, plants, and some animals that causes cellular damage in other organisms
•Toxins are named according to their target- neurotoxins act on the nervous system- enterotoxins act on the intestines- hemotoxins lyse red blood cells- nephrotoxins damage the kidneys
• Two types of toxins in pathogenic bacteria• Exotoxin• Endotoxin
The Progress of an Infection:4. Causing Disease‐Toxins
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Exotoxins• proteins that targets a specific cell
type• affect cells by damaging the cell
membrane and initiating lysis• Don’t confuse with exoenzymes! • Ex: Hemolysins
- disrupt the membrane of red blood cells to release hemoglobin
- Ex. Streptococcus pyogenes and Staphylococcus aureus
The Progress of an Infection:4. Causing Disease‐Exotoxins
• Endotoxin- lipopolysaccharide (LPS), part of the outer membrane of
gram‐negative cell walls- Released when cells die- has a variety of systemic effects on tissues and organs- causes fever, inflammation, hemorrhage, and diarrhea
The Progress of an Infection:4. Causing Disease‐Endotoxins
Some endotoxins are pyrogenic
The Progress of an Infection:4. Causing Disease‐Endotoxins
• Enables pathogen to spread to other hosts– Respiratory
– Salivary
– Skin
– Fecal
– Urogenital
– Blood
The Progress of an Infection:5. Portal of Exit
• specific avenue by which pathogens exit
• shed through secretion, excretion, discharge, or sloughed tissue
• high number of microbes in these materials increases the likelihood that the pathogen will reach other hosts
• portal of exit is usually the same as the portal of entry, but some pathogens use a different route
The Progress of an Infection:5. Portal of Exit
Pathogenicity:The ability to cause disease
Depends on a pathogen’s ability to:
1. Enter
2. Attach
3. Survive host defenses
4. Cause disease
5. Exit
Look familiar?
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Virulence: The degree of pathogenicity
• Intensity of disease produced
• Virulence can be decreased as a pathogen is sub‐cultured time after time
• Many virulence factors (weapons) exist to increase a pathogen’s ability to:1. Enter
2. Attach
3. Survive host defenses
4. Cause disease
5. Exit
• Localized infection: An infection that is limited to a specific part of the body and has local symptoms
• Systemic infection: pathogen is distributed throughout the body
Establishment of Infections
Establishment of Infections
• Focal infection: A bacterial infection localized in a specific part of the body, that spreads to other parts of the body
• Mixed infection: Bacterial infection composed of different species of bacteria
• Primary and secondary infections: Illness caused by new microbe becoming established in the wake of an initial (primary) infection
• Acute: rapid onset of infection, short course of infection
• Chronic: Long duration of infection
• Asymptomatic or subclinical: Not noticed by host
Establishment of Infections
Signs, Symptoms and Syndromes
• Signs• Objective/measurable (I.e., fever, inflammation)
• Symptoms• Subjective (pain, tummy ache)
• Syndromes• Combo of signs and symptoms that occur together
• How would you classify a sore throat? A red throat? A fever?
Stages of an Infectious Disease
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Incubation periods vary
•Recovery of the host does not always mean the microbe has been removed or destroyed by host defenses•Latency: a dormant state of microbes in certain chronic infectious diseases
- viral latency: herpes simplex, herpes zoster, hepatitis B, AIDS, Epstein‐Barr
- bacterial/protozoan latency: syphilis, typhoid fever, tuberculosis, malaria
•Sequelae: long‐term or permanent damage to tissues or organs caused by infectious disease
- meningitis: deafness- strep throat: rheumatic heart disease- Lyme disease: arthritis- polio: paralysis
The Persistence of Microbes and Pathologic Conditions
Epidemiology
• Epidemiology: The study of disease within populations (human, plant, etc.)
• Epidemiology helps us investigate the factors regarding a specific disease:
• what causes a disease
• how is it transmitted
• how do we prevent and treat it
• how many people are afflicted.
Epidemiology
• Epidemiology allows us to take all we have learned about microbes and the diseases they cause and apply that knowledge to new situations.
• Involves many disciplines: microbiology, anatomy, physiology, immunology, medicine, psychology, sociology, ecology, and statistics
Epidemiology Terminology
• Epidemiologists• “disease detectives”, scientists who study epidemiology
• Etiologic agent• The cause of a disease
• Morbidity• Illness
• Mortality• Death
Epidemiology Terminology
• Incidence• Number of NEW cases within a period of time
• Prevalence• TOTAL number of cases within a period of time
• Which one informs us if we have taken proper measures to halt disease transmission?
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Tracking Disease in the Population
•Reportable or notifiable diseases- certain diseases must be reported to authorities
- other diseases are reported on a voluntary basis
•A network of agencies at the local, district, state, national, and international levels keeps track of infectious diseases
Epidemiology TerminologyCommonly reported diseases that are tracked in
the United States
– Endemic– Pathogen is continually present in population
– Sporadic– occasional cases are reported at irregular intervals at random locales
– Epidemic– An “outbreak” or higher than normal number of cases. Ie. prevalence of an endemic or sporadic disease is increasing beyond what is expected for a population
– Pandemic – Spread of an epidemic across continents
Epidemiology Terminology Epidemiology TerminologyEndemic Occurrence Epidemic Occurrence
Pandemic OccurrenceSporadic Occurrence
Cases
Diphtheria cases after the break up of the former Soviet Union.
Is this disease endemic, epidemic, pandemic or sporadic?
Statistical data can be represented graphically, and can be used to predict trends
We can analyze the data according to year, ages affected, and geographic location to help us predict what diseases we need to watch out for and identify precautions to prevent them. (similar to Influenza H5N1)
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Epidemiologic Studies
• Epidemiologists collect data on diseases to help prevent outbreaks in the future.
• Three Types of Epidemiologic Studies:
– Descriptive
– Analytical
– Experimental
Descriptive Studies• Concerned with the physical aspects of an existing disease and disease spread.
• Provides the what, who, when and where
• Records as many details as possible:– Number of cases– Populations affected– Locations and time– Age, gender, race, socioeconomic status, etc.
Examples of Data from Descriptive Studies
First formal epidemiologic study performed by John Snow in 1854. He traced the source of a cholera epidemic to a certain water pump and proved that people became infected by fecally contaminated drinking water.
Analytical Studies• Provides the why and how
• Studies determine causes and factors that influence the rate of disease. These factors include demographic, biological, behavioral, and environmental influences.
• Disease groups are compared to control groups and data is analyzed for similarities and differences.
Example analytical question: in the hanta virus epidemic, did people get sick if they had been outside near mice habitats or if they had been cooped up in office buildings?
Experimental Studies• Designs experiments to test a hypothesis.
• These are the “cleanest” types of studies and often considered the gold‐standard.
• Many experimental studies are performed for pharmaceutical (“clinical trials”) or other treatments.
Disease Transmission
• Disease transmission is affected by:
– Reservoirs of infection
– Portals of Entry and Exit
– Mechanisms of Transmission
– We will investigate all three in order to understand how to break the chain of disease transmission. You can break it at just one site to stop transmission.
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Disease Transmission: Source of Infection
• Reservoir• The natural host or habitat (living or nonliving) of a pathogen
• Source• The person or item from which an infection is DIRECTLY acquired
• Carrier vs. Asymptomatic Carrier• An organism that harbors infections and can spread them to others. They may show symptoms or not.
Disease Transmission: Source of Infection
• Biological vector• An organism which not only transports a pathogen but also plays a role in the life cycle of the pathogen (virus inside of mosquito, bacteria inside of tick)
• Mechanical vector• An organism which only transports a pathogen (fly)
Disease Transmission: Source of Infection
• Zoonosis • An infectious disease humans can acquire from animals (Ex. rabies)
• 70% of all new emerging diseases worldwide
• impossible to eradicate without also eradicating the animal reservoir
• attempts have been made to eradicate mosquitoes and certain rodents
Zoonotic infections are caused by vectors and animal
reservoirs spreading their own infections to humans
Disease Transmission: Source of Infection
• Human Reservoirs• Asymptomatic Carrier
• Symptomatic Carriers
• Animal Reservoirs• Wild animals (Rabies)
• Deer mice (hanta virus)
• Insects
• Nonliving Reservoirs• Soil (Clostridium tetani, Bacillus anthracis)
• Water (Cholera, Giardia)
• Food (E. coli, Salmonella, Listeria)
• Some diseases are communicable but others are not
• Communicable• Infected host transmits an infectious agent to another host
• Receiving host must become infected
• Non‐communicable• Host acquires infectious agent but can’t transmit it to another
– From self (compromised individual)‐ microflora
– Nonliving reservoir – soil‐(tetanus)
Disease Transmission: Source of Infection
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• Communicable
– Disease spread from one host to another
• I.e., cold, meningitis
• Contagious
– Easily communicable
• I.e. measles, influenza
– Are all communicable diseases contagious?
– Are all contagious disease communicable?
Disease Transmission: Source of Infection Disease Transmission: Patterns of Transmission
• Horizontal transmission• Disease is spread through a population from one infected person to another
– Kissing, sneezing
• Vertical transmission• The disease is transmitted from parent to offspring
– Ovum, sperm, placenta, milk
Disease Transmission: Patterns of Transmission
• Direct (contact)• Kissing, sex• Droplets (sneezing, coughing directly upon a person within 3 feet)
• Vertical• Vector
• Indirect– Contaminated materials
• Vehicles=Food, water, biological products (blood, serum, tissue), fomite (door knobs, toilet seats, etc.)
• Fecal‐oral (aka oral‐fecal)
– Air (greater than 3 feet away)• Droplet nuclei (dried microscopic residue)
• Aerosols (dust or moisture particles)
A sneeze can release enormous amounts of moist droplets, and the dry droplets form droplet nuclei. (so cover your
mouth with your elbow )
Disease Transmission: Prevention
• Sanitation
– What is the #1 way to stop disease?
• Immunization
• Isolation
• Quarantine
• Control vectors
• Education about prevention and treatment
Disease Transmission: Prevention• Herd immunity is the proportion of people immune to a certain
disease. Is it easier or more difficult to transmit disease when herd immunity is low? How do we acquire herd immunity?
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Nosocomial Infections: The Hospital as the Source of Disease
• Nosocomial Infection: An infection acquired in a hospital
• About 2‐4 million (5‐20 percent) of admitted patients acquire a nosocomial infection
• 90, 000 die of nosocomial infections
• $5‐10 billion per year to treat nosocomial infections
- compromised patients
- collection point for pathogens
- lowered defenses permit normal biota to enter the body
- infections acquired directly or indirectly from fomites, medical equipment, other patients, medical personnel, visitors, air, and water
Nosocomial Infections: Contributing Factors
The most common nosocomial infections
Common nosocomial pathogens
•Healthcare processes that lead to nosocomial infections:
• treatments using reusable instruments such as respirators and thermometers
• indwelling devices such as catheters, prosthetic heart valves, grafts, drainage tubes, and tracheostomy tubes form ready portals of entry
• high proportion of the hospital population receives antimicrobial therapy, drug‐resistant microbes are selected for at a much higher rate
Nosocomial Infections
• Recent evidence suggests that more than 1/3 of nosocomial infections could be avoided by consistent and rigorous infection control methods
• Use Universal Precautions (aka Standard Precautions)
• Assume all patients and fomites may harbor pathogens
Nosocomial Infections: Prevention and Control